The invention relates to an apparatus for measuring the flow of a medium to be measured which flows through a measuring tube in the direction of the measuring tube axis, having a magnetic arrangement which generates a magnetic field which permeates the measuring tube and runs essentially transversely with respect to the measuring tube axis, having at least one measuring electrode which is arranged in a lateral region of the measuring tube and is coupled electrically or capacitively to the medium to be measured, and having an evaluation/control unit which, using the measurement voltage induced in the measuring electrode, provides information about the volumetric flow of the medium to be measured in the measuring tube.
Electromagnetic flowmeters utilize the principle of electrodynamic induction for volumetric flow measurement: charge carriers of the medium to be measured which are moved perpendicularly to a magnetic field induce a voltage in measuring electrodes which are likewise arranged essentially perpendicularly to the flow direction of the medium to be measured. This induced voltage is proportional to the flow velocity of the medium to be measured averaged over the cross section of the tube; it is thus proportional to the volumetric flow.
Considerable measurement errors can occur if the measuring tube is not filled completely but only partially with the medium to be measured, yet the measuring apparatus bases the measuring results that are yielded by it on a completely filled measuring tube. In order to eliminate this source of errors, apparatuses have already been disclosed which enable the information about the respective degree of filling of the measuring tube to be taken into account in the determination of the volumetric flow. Thus, German utility model G 91 03 046.3 has described a electromagnetic flow-measuring apparatus in which two electromagnetics are optionally excited in each case individually or jointly and where the joint excitation may optionally involve excitation in the same sense or excitation in opposite senses. In order to determine the volumetric flow, use is made of at least two voltage values which are tapped off by a corresponding pair of measuring electrodes and have been measured for different excitation states of the electromagnetic arrangement, for example excitation of the two electromagnets in the same sense and in opposite senses. An evaluation unit subsequently processes the measuring signals by means of parameters that are determined empirically to form a flow rate output signal, in the case of which the errors on account of partial filling of the measuring tube are eliminated. In order to ensure that even when the degree of filling of the measuring tube is extremely low, at least one pair of electrodes is electrically coupled to the medium to be measured and can be used for providing the measurement signal, the flow-measuring apparatus described in G 91 03 046.3 has two pairs of measuring electrodes arranged one above the other in the lower region of the measuring-tube cross section.
Furthermore, the prior art has disclosed the provision, in addition to two measuring electrodes arranged in the central region of the measuring tube, in each case of a test electrode in the upper region and a further test electrode in the lower region of the measuring tube, the lower test electrode usually being grounded. While the voltage measured at the test electrodes serves for identifying the degree of filling of the measuring tube, the desired information about the volumetric flow through the measuring tube can be obtained from the voltage values measured at the measuring electrodes.
In order that the known apparatus for determining the degree of filling of the measuring tube can be used as universally as possible; the source resistance of the test voltage applied to the test electrodes must be relatively high (e.g. of the order of magnitude of 100 kxcexa9). It is only in this way that the apparatus for determining the volumetric flow can be used universally for the large range of media to be measured which have to be covered and, as is known, can differ quite considerably from one another in respect of their electrical conductivity.
The disadvantage of the known solution is manifested when, instead of a compact device in which the actual sensor and the electronic section are accommodated in a housing, a measuring device is used in which the sensor is arranged in the process and is connected to remote conversion electronics via a connecting line, usually a coaxial cable. This connecting line brings about voltage division dependent on the length of the coaxial cable, where a measurable voltage component is no longer available at all at the test electrode when the connecting line is more than a certain length. Consequently, the known measuring apparatus for identifying the degree of filling of the measuring tube is restricted to certain applicationsxe2x80x94namely to use in compact devices or in devices in which the connecting line between the sensor and the remote electronic unit does not exceed approximately 10 m.
The invention is based on the object of proposing a universally usable and cost-effective apparatus for measuring the volumetric flow and/or for identifying the degree of filling of a measuring tube.
The object is achieved by virtue of the fact that at least one test electrode is provided in the upper region of the measuring tube, that the evaluation/control unit passes a test signal to the test electrode and that the evaluation/control unit uses the response signal to the test signal, said response signal being received by the measuring electrode, to provide information about the degree of filling of the measuring tube. According to the invention, the so-called xe2x80x9cempty tube detectionxe2x80x9d, that is to say the identification of whether the measuring tube is completely filled or only partially filled or entirely empty, is effected by the application of a test signal to the test electrode. If the measuring tube is filled, then said test signal is manifested as a response signal at the measuring electrode. If, on the other hand, the measuring tube is only partially filled or if it is empty, then there is no electrical connection, or there is a disturbed electrical connection, between the top test electrode and the measuring electrode. Consequently, no response signal or else an attenuated response signal appears at the measuring electrode.
Although, in principle, one measuring electrode is sufficient for determining the volumetric flow and for identifying the xe2x80x9cempty tube detectionxe2x80x9d, an advantageous embodiment of the apparatus according to the invention provides a second measuring electrode, which is arranged in the region of the measuring tube opposite the first measuring electrode. The two measuring electrodes are preferably diametrically opposite one another in the central region of the measuring tube.
In order to utilize conditions of symmetry and redundant measurement that is enabled as a result of this, the test electrode is arranged with respect to the two measuring electrodes in such a way that it is essentially at the same distance from each of the two measuring electrodes. A test signal output by the test electrode thus generates an essentially identical response signal at each of the two measuring electrodes. If serious differences occur in this case, then this indicates, under certain circumstances, a malfunction, e.g. of one of the measuring electrodes.
An advantageous development of the apparatus according to the invention provides a second test electrode, which is essentially diametrically opposite the first test electrode, the first test electrode preferably being arranged in the topmost point of the measuring tube and the second test electrode in the bottommost point of the measuring tube. One refinement of the apparatus according to the invention proposes that one of the two test electrodes is grounded. The grounded test electrode is preferably the electrode which is arranged in the lower region of the measuring tube. As already indicated above, the second test electrode used as a reference electrode is not absolutely necessary. Grounding disks may also serve as reference potential, said disks being provided for example on the flanges for fixing the flow-metering apparatus in a tube system.
A preferred development of the apparatus according to the invention suggests that the test signal be a symmetrical pulse. This embodiment has the following advantage over an arbitrary non-symmetrical test signal: the relatively high energy density of the test signal can lead to a shift in the electrochemical potentials within the medium to be measured. If the test signal is designed as a symmetrical pulse, then the potential shifts average out approximately to zero.
A preferred embodiment of the apparatus according to the invention provides for the evaluation/control unit to correlate the response signals at the test electrode and the measuring electrode/the measuring electrodes with the test signal. This is a simple and reliable method for identifying whether or not the response signal to the test signal appears at the measuring electrode/at the measuring electrodes.
Furthermore, an advantageous development of the apparatus according to the invention provides for the evaluation/control unit to determine the information about the degree of filling by comparing the response signals measured at the test electrode and/or at the measuring electrode/the measuring electrodes with predetermined reference signals. These reference signals are determined beforehand in a separate measuring process under the respectively prevailing process and system conditions. The tolerances can thereby be designed in an apparatus-related manner and, consequently, such that they are relatively small, as a result of which the quality of the measurements can be improved and reliable identification of an incompletely filled measuring tube can thus be achieved.
A preferred development of the apparatus according to the invention proposes a storage unit, in which the predetermined reference signals are stored.